How-to: Carb Synching

posted Feb 24th, 2009 at 4:47pm

Suddenly, your bike runs like hell. It's hard to start, idles erratically and feels down on power. What you need is a good sync, a carb sync that is. What's that? On multicylinder bikes with separate carburetors (or fuel-injection throttle bodies), all carbs need to be doing their fair share and operating in tight lockstep with one another. When one or more carburetors gets out of line, some cylinders will be receiving more air and fuel than the others, causing uneven idling and, possibly, surging at cruising speeds.

Maintenance technicians take training classes in the art of carb syncing, but it's a pretty straightforward exercise requiring only the right tools and some patience. First, a few cautions: Have a clean, well-lit workspace with good ventilation; you'll be running the engine during syncing so don't close yourself in the garage. Also, you'll need a reasonably powerful fan to help keep the engine cool 1.; a lot of shadetree wrenches do without, but if the engine overheats your settings won't be right.

About those tools: You'll need a carb balancer, of course. Several types are available, including some inexpensive mercury manometers (tall glass or plastic tubes attached to a reservoir of mercury on one end and hoses that lead to the intake ports on the other), individual vacuum gauges and spiffy (but pricey) electronic items. We happened to have the purely mechanical Carbtune on hand. Plan to buy or fabricate a temporary fuel source, because on most bikes you'll have to do the syncing with the tank removed. You can also use the stock tank elevated and fitted with a long fuel line. Then find a long, thin Phillips screwdriver in that toolbox of yours.

Start by clearing away as much bodywork as you can to gain access to the carbs 2..(Don't worry, this is the hardest part.) Some bikes will have screws filling ports in the intake tract; the most common size is 5mm, but some Yamahas use 6mm threads. Make sure your balancer has the right adapters. Still other bikes have nipples on the intake tubes, like the Bandit shown here 3.. Remove any lines or covers and attach the carb balancer lines, keeping track of the order; left-most cylinder to the left-most tube, and so on. Check to make sure the hoses don't get in the way of the throttle linkages or cables. See if the idle-mixture fuel (or air) screws are exposed and double-check that they're set correctly and equally.

With the temporary fuel supply attached, start the engine and let it warm up. Resist the temptation to blip the throttle because the spike in manifold vacuum can draw the mercury out of the balancer. Look down between the carbs and you'll see a small screw head near the common throttle shaft. There are three of these on a typical four-cylinder bike to set the relationship of the adjoining carbs. Begin by adjusting the screw between the number-one and number-two cylinders to match the readings on the balancer 4.. Normally, the spec says to get them within 0.5 to 1.0 in. of Hg of each other but there's nothing wrong with setting them dead even. Gently blip the throttle to make sure the linkage has taken a set and check the balancer. Next, move to the number-three and number-four pair and balance them 5.. Finally, use the center adjustment to synchronize the left and right pairs. By now, you may have to reset the idle speed 6.. Don't be surprised if you have to go back and slightly tweak the settings one more time.

As a final test, run the engine up to a low cruising rpm and see how the vacuum signals compare. (If they're way off now but fairly equal at idle, you may have a sticky slide, sloppy linkages or some other malady and syncing won't help.) Shut off the engine, remove the test gear and button it up, being careful to secure the manifold plugs and fuel connections.